Oxide ion transport number

A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

The perovskite structure is capable of high anion conductivity when oxide vacancies are introduced, as in, for example, Lai (Sr Co03 (/2 or in the perovskite-related superconductor phases, La2Cu04 and YBa2Cu307. The oxide ion transport number is not unity since such materials are often electronic conductors as well, due to the presence of... 

Oxide ion transport number, tO2 O, Nd2Eu203F6 , Nd2Ce203F6 A, Nd2Gd203F6 ... 

Fig. 23. The transport number of various rare-earth fluoride stabilized zirconias. Oxide ion transport number, tO2 --------- Electron transport number, re-------- ...

On the other hand, there are few reports about SOFC stacks, which employ rare earth doped ceria electrolytes, although a high power density has been reported for single cell test (Steele, 2000 Bance et al., 2004). The rare earth doped ceria exhibits isothermal expansion in a reducing atmosphere due to the reduction of cerium ion from tetiavalent (Ce +) to trivalent state (Ce " ") accompaitying the formation of oxygen vacancies, which results in the warping or de-straction of the electrolyte plate. The formation of trivalent cerium ion also causes a decrease of oxide ion transport number, which reduces the efficiency of the cell. 

However, the oxide ion transport number also needs to be evaluated as well as the conductivity when the materials are considered as SOFC electrolytes. As already pointed out in the section 2.1, the reduction of cerium ion in RDC occirrs in a reducing atmosphere, for example ... 

The high conductivity of cerium-lanthanum mixed oxides and the favourable polarisability of electrodes on such solid electrolytes was already stimulating application ideas in the 1960s. But electronic conductivity of these electrolytes above 600°C was seen as a weighty problem [71]. The influence of electronic conductivity on the cell performance was investigated first by means of an equivalent circuit [40,100]. The results, shown in Figure 2.8, led to the conclusion that the ion transport number has to be greater than 0.9 if a solid electrolyte was to be successful in a SOFC [100]. 

AU these features—low values of a, a strong temperature dependence, and the effect of impurities—are reminiscent of the behavior of p- and n-type semiconductors. By analogy, we can consider these compounds as ionic semiconductors with intrinsic or impurity-type conduction. As a rule (although not always), ionic semiconductors have unipolar conduction, due to ions of one sign. Thus, in compounds AgBr, PbCl2, and others, the cation transport number is close to unity. In the mixed oxide ZrOj-nYjOj, pure 0 anion conduction t = 1) is observed. 

Radiotracer techniques involving lsO in the anodization process are used with subsequent neutron activation analysis84 or SIMS.85 Another method involves implantation of inert ion markers into the surface layer of the sample prior to anodization and examination of the position of the markers after the oxide film has grown to a certain thickness.86 Assuming immobility of the inert species, the ratio of the cation to the anion transport number, t+/, should be equal to the ratio of the outer to the inner layer thickness. Numerous experimental determinations72,87 suggest t+ and f to be 0.4 and 0.6, respectively. 

Takahashi et a/.,79 in their work on the structure of the barrier layer [cf. Section IV(2)], have considered phosphate ions, which are found in the outer layer of the oxide, as immobile markers and, from the position of the boundary between the outer and the inner layer, deduced the transport number of the cation to vary between 0.73 and 0.81 in the current density range between 0.05 and 10 mA/cm 2. 

Figure 6.2 Ionic transport number for oxide ion conductivity in the pyrochlore phases Lu2Ti207, Lu2.096Tii.904O6.952> and Lu2286Tii.7i406.857- [Data adapted from A. V. Shlyakhtina, J. C. C. Abrantes, A. V. Levchenko, A. V. Knot ko, O. K. Karyagina, and L. G. Shcherbakova, Solid State Ionics, 177, 1149-1155 (2006).]...

A number of researchers, ° ° particularly in Japan, have been pursuing the oxides of iron as potential cathode materials for lithium cells. However, materials of the type LiFeOz have shown little ability for lithium removal. A number of other iron compounds have been studied over the years, including FeOCl, ° FePS3, ° KFeSz, and FeSz, but none showed much reversibility. Although metal phosphates have been studied for more than 20 years since the discovery of fast ion transport in NASICON, it is only recently that they have been considered as cathodes or anodes " of lithium batteries. 

This parabolic law, which indicates that diffusion is rate-limiting, is of overwhelming importance for scale formation. Wagner (1933) showed that the parabolic scale constant (and hence, rate of oxidation) can be calculated using the enthalpy of formation of the corrosion product, the electrical conductivity of the protective film and the transport number of the ions and electrons in the film. 

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